The present invention relates to amplifiers and more particularly to high power, high frequency amplifiers employing feedforward control for improved linearity.
High power amplifiers for use in communications and other environments are requiring improved performance in terms of power amplifier linearity and bandwidth. While certain high performance systems presently utilize vacuum tube technology, the same is not compatible with changing communications environments requiring solid state power amplifier designs to reduce size, weight and cost as well as to allow greater network simplicity and improved reliability. However, before such conventional vacuum tube systems can be replaced with solid state designs, there must be significant improvements in the characteristics of such solid state amplifier circuits in terms of improvements in intermodulation distortion and transmitted noise performance.
By way of example, current linear power amplifiers constructed for use in high frequency environments have limited performance because of device non-linearities. In particular, such non-linearities introduce distortion into the output of the power amplifier and some technique must be used to reduce intermodulation distortion to acceptable levels before the linear power amplifiers will attain high versatility. Accordingly, two known methods of reducing such intermodulation distortion include feedback and feedforward techniques.
As is known in the art, both of the above methods have inherent drawbacks which limit their applicability to linear power amplifiers. Specifically, in the case of feedback systems, the greatest problem in implementing the amplifier is in maintaining its stability under a variety of operating conditions. More particularly, because a feedback system is correcting for an error that has already occurred in an earlier timeframe, acceptable results can only be achieved at the expense of increased gain and bandwidth requirements. Such circuits therefore sacrifice efficiency and bandwidth to achieve distortion reduction for use in high power and high frequency systems.
In other instances, the intermodulation distortion problem has been addressed by using a feedforward concept. This concept is well known and is described in such U.S. Pat. Nos. as 1,686,792; 2,102,671; 3,471,798; and more recently in U.S. Pat. No. 4,352,072, assigned to the same assignee as the present invention each of which is incorporated by reference herein in its entirety. The operation of such circuits to reduce intermodulation distortion and provide improved stability, as well as the elements used to implement such systems, is clearly referenced in the above-identified patents and additionally in the article entitled "Feedforward--An Alternative Approach to Amplifier Linearization" by T. J. Bennett and R. F. Clements in the Radio and Electronic Engineer, Vol. 44, No. 5, May 1974. Such systems apply error correction without feedback and accordingly provide improved linearity by compensating in the same timeframe that any distortion within the amplifier is created.
In such feedforward systems, the error correction is accomplished by sampling the distorted output from the main amplifier and removing the undistorted input from that output through a hybrid junction. The resulting output from the hybrid junction then includes only the distortion products created by the main amplifier. The level of the distortion products is then raised to an appropriate level by an error amplifier and subtracted from the main amplifier output by an injection coupler. The result is an output signal amplified by the main amplifier with the distortion products subtracted from that main amplifier output during the same timeframe. In operation, the signal delays in all parallel paths are equalized by the use of phase equalizers and coaxial lengths having the same delay characteristics in both the main amplifier and error amplifier loops. This thus removes the distortion products in the same timeframe that the distortion is created without requiring direct feedback paths which would render the system unstable under varying operating conditions.
While such feedforward systems as are described in the prior art provide improved performance at low power levels, the same have limitations when implemented in systems to be used at higher power levels. Such problems are basically caused by the delays encountered resulting from the multiple stage amplification of the main and error amplifiers, and the phase and amplitude linearity requirements over frequency for the main and error amplifiers. In particular, the delay lines used in such prior art feedforward systems take the form of lengths of coaxial cable which typically may be constructed as a thirty-foot length of RG-213 coax. This cable will produce a 30 watt loss in a 500 watt system and proves to be bulky and space-consuming, contrary to the requirements of current communications equipment. Furthermore, in order to maintain phase and amplitude linearity over frequency, high power phase equalizers of increased complexity in design and implementation must be used. This tends to be expensive and complex and reduces the overall desirability of such high power systems requiring phase equalization.
In order to overcome the above problems, a system and technique is proposed in a co-pending application Ser. No. 615,500 entitled "A Phase Adjusted Feedforward Error Correction Circuit" by E. G. Silagi and M. W. Heidt, filed on even date herewith and assigned to the same assignee, which application is hereby incorporated by reference in its entirety. Although such a system improves the performance of feedforward circuits, there are still an unlimited number of practically realizable phase shift networks which may be used to produce the phase shift necessary in the feedforward systems of the aforementioned application. More specifically, the need for variable phase shift with constant impedance and amplitude response is necessary under electronic control in order to implement the improved circuits while reducing cost, weight, and complexity, while improving reliability. This realization becomes more difficult as the bandwidth of operation increases, for example greater than one octave.
For implementing such phase shift circuits in the high frequency spectrum of 2-30 MHz (nearly four octaves), the practical implementation thus becomes more difficult. One method of realizing a constant impedance all-pass network is by the use of a conventional lattice network including inductors and capacitors which are electronically variable. However, while the phase shifted frequency range may be physically realizable in certain ranges of operation, the same may not be true for all frequency and phase shift ranges necessary for operating in the high frequency spectrum. In addition, more complicated control circuitry is necessary to make the circuit work over such broad frequency ranges. Accordingly, the network described in the co-pending application entitled "Phase Shift Network with Minimum Amplitude Ripple" by E. G. Silagi, filed on even date herewith and assigned to the same assignee as the present application (which application is incorporated herein by reference in its entirety) has been proposed to more effectively provide phase shift in feedforward systems.
Although the improved feedforward error correction system and the improved phase shift network described in the aforementioned co-pending applications provide improved feedforward performance by reducing the complexity and improving the power loss problems in contrast to the prior art, the number of phase shift networks is still equal to that of the prior art. In addition, the need for active control of the phase equalizers requires additional control circuitry to maintain proper response over the range of frequency at which the system operates. Accordingly, there is still a need to provide feedforward systems of reduced complexity while still accomplishing the broad objectives of feedforward control.
Accordingly, the present invention has been developed to overcome the specific shortcomings of the above known and similar techniques and to provide a feedforward system having a reduced number of equalizers for system operation.